The present invention relates to a furnace firing apparatus and method for burning pulverized fuel and, more particularly, to such an apparatus and method for use in large arch firing units or for use in burning low volatile fuel.
Over the years a wide variety of burner and furnace designs have been developed for handling and burning pulverized fuels. In a typical coal-fired furnace, pulverized coal, suspended in primary air, is delivered from a pulverizer, or mill, to the coal burners, or nozzles, and secondary air is provided to supply a sufficient amount of oxygen to support combustion. After initial ignition by a high energy arc igniter or small oil or gas conventional gun igniter, the subsequent incoming coal is ignited by recirculating a portion of the hot gases, generated from the combustion of previously introduced coal, into the incoming fuel stream.
Low volatile fuels, such as anthracite, antracite silt and petroleum coke, have less than one-third of the volatile matter of other fuels, and they require more time to ignite and longer time for complete, or near complete, combustion. The self-sustaining method as described above results in an inefficient method of burning low volatile fuels since a relatively large amount of the fuel will remain unconsumed, unless an arch unit is utilized. In an arch unit, this self-sustaining flame is produced by down-firing the coal into the furnace and introducing secondary air further down. This process can be enhanced by using conventional cyclone burners to introduce the fuel into the furnace with less suspension air.
To increase the percentage of low volatile fuel which can be consumed in arch fired furnaces, the length of the arch can be increased to subject the fuel to a longer burn time. However, there are physical and economical limits to a furnace's arch length. When these limits are reached, multiple arches are required. Lining a furnace with multiple arches, however, significantly increases the cost of both building and operating the furnace since each arch requires fuel and air inlets and initial ignition by conventional igniters.